This invention relates to an improved closure or diaphragm for offshore platforms used in well drilling and production.
Offshore platforms are generally fabricated in a harbor or on a shore location and are then towed to a marine site where they are tipped on end and lowered into position with the platform resting on the ocean floor. The platform legs are hollow structures having open ends so that pilings can be driven downwardly through the legs into the subterranean formations below the ocean floor to anchor the platform into position.
It is desirable during platform setting operations to utilize the platform legs and/or pile sleeves for bouyancy to assist in the setting operations. It is also desirable to exclude foreign material from the platform leg and/or pile sleeve during platform setting operations to prevent the annulus between the piling and the platform leg and/or pile sleeve from becoming contaminated with foreign material which would prevent the filling of the annulus with cement or grout. Therefore, a closure structure which is easily severable when the piling is driven through the platform leg and/or pile sleeve is used to seal the end of the platform leg and/or pile sleeve during setting of the platform.
Typical prior art closure structures, generally referred to as closures or diaphragms, utilized to seal the end of a platform and/or pile sleeve of an offshore platform, are illustrated in U.S. Pat. Nos. 3,533,241, 4,024,723, 4,178,112, 4,220,422, and 4,230,424. While these closures are generally satisfactory, all utilize layers or plies of reinforcing material comprised of unitary pieces of fabric to cover the closure.
Another prior art closure or diaphragm shown in U.S. Pat. No. 4,367,983, utilizes layers or plies of reinforcing material comprised of strips of fabric where each strip is oriented with respect to another strip in the same layer or ply and with another in an adjacent layer or ply such that the number of longitudinal threads of the fabric of the strips of each layer or ply generally extend along radial lines in a horizontal plane of the closure.
Yet another prior art closure or diaphragm, also utilizing layers or plies of reinforcing material comprised of strips of fabric is disclosed in U.S. Pat. No. 4,470,726. Unlike the closure or diaphragm disclosed in U.S. Pat. No. 4,367,983, the latter closure or diaphragm comprises a plurality of strips of fabric, at least one strip of fabric having the ends wrapped and secured about one or more reinforcing members in the outer periphery of the closure or diaphragm and covering the center of the closure or diaphragm and the remaining strips of fabric having each end thereof wrapped and secured about one or more reinforcing members in the outer periphery and extending along lines which are chords of a circle formed by a reinforcing member such that the closure or diaphragm has each area thereof reinforced by at least one strip of fabric.
While the closures or diaphragms shown in both U.S. Pat. Nos. 4,367,983 and 4,470,726 are believed to constitute improvements over the closures or diaphragms employing unitary pieces of fabric as disclosed in the previously mentioned patents, such closures or diaphragms still suffer from problems resulting from the manner in which the strips of fabric are overlapped around a peripheral reinforcing member at the outer edge of the closure or diaphragm. Specifically, the prior art method of overlapping fabric about a peripheral reinforcing member or shim results in all of the fabric overlaps being on one side ("side" referring in this instance to the top or bottom) of the reinforcing member and all of the primary load carrying strips extending across the face of the closure or diaphragm being on the other side of the reinforcing member. This is particularly evident in the '726 patent, wherein only one reinforcing member is employed. However, the same disadvantage obtains with the '983 patent, wherein two reinforcing members are employed but the fabric strip overlaps still remain on one side of a reinforcing member, while the primary load carrying strips remain on the other side of that reinforcing member. With either prior art design, the overlap arrangement causes the reinforcing member to twist when the closure or diaphragm is stressed. This twisting moment, or torque, occurs because the horizontal force exerted by the fabric strips on the reinforcing member does not lie in the same horizontal plane as the center line of the reinforcing member. In some instances, this torque is great enough to permanently deform the reinforcing member when the closure or diaphragm is pressure tested, making it extremely difficult to assemble the closure or diaphragm with the annular flanges used to secure same to the bottom of a platform leg or pile sleeve and align the bolts extending through the closure or diaphragm with the flanges on either side thereof in order to clamp the edge of the closure or diaphragm between the aforesaid flanges. An additional problem of the prior art overlap arrangement, due to the fact that all of the overlaps about a reinforcing member are all on the same side thereof, is that the overlap bond to the primary loading carrying strips sometimes fails or pulls loose when the closure is pressure tested. This phenomenon occurs because the bond area between the first, or innermost overlaps and the primary strips is relatively small, and each successive overlap layer wrapped around the reinforcing member over the first overlaps is bonded primarily to the first or another preceding overlap and not to a primary load carrying strip.
The aforementioned U.S. Pat. No. 3,533,241, which employs unitary plies of reinforcing material rather than the preferred strip-type plies of the '983 and '726 patents, does employ an alternating wrapping direction for each successive ply about a peripheral reinforcing member. However, the overlap edges of these unitary plies are not extended back over the face of the diaphragm, so that the ply/ply bond area is severely limited and takes place about a reinforcing member of round cross-section, precluding any assurance of continuous contact (no voids) between adjacent plies and subjecting the arcuate ply/ply bond areas to different shear loads in the bond area when the closure or diaphragm is tested or used, due to the varying diameters of the overlapped ply edges. Furthermore, the non-extension of the overlap edges back over the face of the diaphragm means that the number of plies inward of the reinforcing member is limited to the number extending across the diaphragm face even in the high stress area immediately inward of and adjacent to the reinforcing member. In addition, the round cross-sectional shape of the reinforcing member requires costly grooved plates to secure the diaphragm to the platform legs, and the close proximity of the overlapped edges to the outside edge of the diaphragm increases the risk that the plies at that location will be pinched or cut when the diaphragm is press-cured secured between the aforesaid plates.